1. Field of the Invention
This invention relates to a process for high yield manufacture of high purity melamine and the apparatus to carry out the process.
More particularly this invention is directed to melamine preparation starting from urea using a high pressure process.
2. Description of Prior Art
Melamine is presently manufactured from urea according to the following simplified reaction scheme: ##STR1##
The reaction is highly endothermic, the reaction heat at a temperature of 360 to 420.degree. C. being approximately 93,000 Kcalories per Kmole of melamine.
The process can be carried out at low pressure in presence of catalysts or at high pressure without any catalyst.
Both low pressure and high pressure melamine manufacturing processes starting from urea are believed to proceed through a series of intermediate reactions leading to, respectively, isocyanic acid, cyanuric acid, ammelide, ammeline and finally melamine. It seems that the following reactions are involved: ##STR2##
By summing up equations 1 to 6 the overall reaction of equation (A) is obtained.
Some of these intermediate products, namely ammeline and ammelide, hereinafter collectively referred to as OAT (OxyAminoTriazine), have been detected in the reaction products.
Moreover melamine, obtained as above, reacts with itself under reaction temperature and pressure yielding so-called polycondensates and a release of ammonia. Polycondensates, beyond being impurities reducing melamine purity degree, also decrease overall reaction yields.
Polycondensates result from amine group (--NH.sub.2) ammonolysis of the melamine molecule and are formed, for example, according to the following ##STR3##
The above reactions are promoted by a low or nonexistent ammonia partial pressure and the long residence time of melamine in the liquid phase (.gtoreq.355.degree. C.). Under the melamine synthesis conditions, polycondensates are obtained in a low amount; however such an amount is not negligible with respect to the final product purity. Anyhow an almost complete polycondensate to melamine regression is achieved by increasing ammonia partial pressure. In conventional melamine synthesis processes, polycondensates conversion to melamine takes place in the melamine purification section wherein, inter alia, an ammonia treatment of the reaction product is provided for.
In the high pressure process, molten urea at a temperature of 140 to 150.degree. C. is fed to a reactor, kept at a temperature of 360 to 420.degree. C. by means of suitable heating devices. In this reactor, molten urea mixes with melamine and remains under stirring actions of the evolving reaction gases for a determined period of time. Raw melamine product is subject to a purification treatment, for instance by dissolving it in water, and subsequent recrystallization to eliminate unreacted urea and remove reaction by-products essentially consisting of reaction gaseous products (ammonia and carbon dioxide), liquid reaction products essentially comprising OAT (mainly ammeline) and polycondensates.
In the industrial process embodiments, reaction is carried out in continuous manner, typically in a single reactor consisting of a cylindrical vessel (tank reactor) wherein the reactants are kept under vigorous mixing by the generation and evolution of reaction gaseous products. Reaction heat is supplied to reactants through suitable heat exchange tubes in which molten salts circulate at a temperature higher than the reaction temperature.
Inside the reactor, each chemical species concentration exhibits a constant value almost in any point of the liquid reaction mixture. Molten urea, continuously fed to the reaction zone, immediately mixes with circulating, reaction mixture. The reaction product is continuously removed and it has the same concentration as the reaction mixture in the reactor. In said reaction arrangement, the higher is the desired conversion rate, and the lower is the melamine production rate. Therefore large reaction volumes are required resulting in a very expensive operation in that the reactor has to be resistant to the highly corrosive action of the reactants and reaction products under very severe temperature and pressure conditions. As a consequence, the costs of the material of the reactor and its working are extremely high.
Even if the reactor had a reactor volume sufficient to achieve approximately a 100% conversion ratio, by remarkably increasing in such a way the reactor costs, it would not be possible to manufacture melamine at a purity degree as required by the market. As a matter of fact, on one side, even optimizing the mixing of reaction mixture, it is not possible to prevent part of the reactants (urea) from coming out of the reactor before the necessary residence time is elapsed to enable its complete dissolution into the liquid mass and its complete conversion to melamine. The smaller is the reaction volume the more is the content of unreacted components which is present in the reaction mixture. Moreover unreacted component content increases, depending on the departing of the reaction mixture from the ideal mixing conditions. On the other side, the residence time distribution in the reaction is such that roughly one half of the reactant mixture remains inside the reactor for a period of time longer than the average residence time, i.e. the ratio between the reactor volume and reactant volume flow rate. Since the reaction mixture practically consists of melamine only, it is subject for a long period of time to ammonolysis reaction resulting in an increased amount of polycondensates.
Therefore, single reactor melamine manufacturing processes yield low purity degree melamine (lower than 97-98%) suitable for marginal uses only, unless the reaction product is submitted to purification treatments affecting the process overall economy, to reach high purity melamine (higher than 99.5%).
Multiple reaction section melamine synthesis processes have been proposed which allow increases in melamine purity. An example of a two-step melamine synthesis process has been disclosed in U.S. Pat. No. 3,116,294. However, since the second reactor employed in the second step is analogous to the first, i.e. both are tank reactors, same drawbacks are experienced, even of a less importance, as in the single reactor process.